Communication systems with automatic retransmission query (ARQ) functionality allow a receiver to indicate to a transmitter that a given data packet was received with errors and should therefore be retransmitted. ARQ techniques are also referred to as automatic repeat request techniques, and are described generally at pp. 259-260 of B. Sklar, "Digital Communications: Fundamentals and Applications," Prentice Hall, Englewood Cliffs, N.J., 1988, which is incorporated by reference herein. A conventional ARQ technique known as stop-and-wait ARQ is suitable for use with half-duplex transmission channels. In stop-and-wait ARQ, a receiver sends an acknowledge (ACK) signal to a transmitter after a given packet is received successfully. The transmitter waits until the ACK signal is received for the given packet before transmitting the next packet in a sequence of packets to be transmitted. If the receiver detects an error in a given packet, it sends a negative acknowledge (NAK) signal to the transmitter, and the transmitter then retransmits the given packet. Other ARQ techniques suitable for use with full duplex transmission channels include continuous ARQ with pullback and continuous ARQ with selective repeat. In both of these techniques, after transmitting a given packet, the transmitter continues to transmit additional packets in the packet sequence even though an ACK signal has not yet been received for the given packet. If the receiver sends a NAK signal indicating that the given packet needs to be retransmitted, a transmitter implementing continuous ARQ with pullback responds to the NAK signal by returning to the given packet in the sequence, and retransmitting the given packet and all subsequent packets which were transmitted prior to receiving the NAK signal for the given packet. A transmitter implementing continuous ARQ with selective repeat also retransmits the given packet, but then picks up in the sequence where it left off before receiving the NAK signal, such that packets following the given packet are not retransmitted unless NAK signals are also received for those packets.
These and other conventional ARQ techniques may be used in place of or in conjunction with error correction techniques such as forward error correction (FEC). ARQ techniques which are used in conjunction with FEC are referred to as hybrid ARQ techniques and include Hybrid Type I and Hybrid Type II techniques. A Hybrid Type I ARQ technique includes some amount of FEC capability in each transmitted packet, and if a retransmission of a given packet is required, retransmits the same version of the packet which was previously transmitted. An advantage of this approach is that additional circuitry for encoding and decoding different packet versions is not needed because the same version of the given packet is used for retransmission. A Hybrid Type II ARQ technique may also include some FEC capability with each packet, but retransmits a different version of a given packet if retransmission of that packet is requested. The Hybrid Type II approach provides advantages in that if the characteristics of the data packet led to the errors, the retransmitted version may not be susceptible to the same errors. Unfortunately, the flexibility of Hybrid Type I and Type II ARQ techniques is limited in that each technique requires a certain type of retransmission that may not be suitable for all applications. A system designer is generally required to select a particular one of the hybrid techniques for use under all operating conditions even though the other technique might provide superior performance under certain conditions. The current inflexibility of the available Hybrid Type I and Type II ARQ approaches may therefore lead to an excessive number of packet retransmissions and a corresponding decrease in the capacity of the communication system.
As is apparent from the above, a need exists for an improved ARQ technique which can provide the beneficial features of both conventional Hybrid Type I and Type II ARQ techniques while avoiding the above-noted problems associated with such techniques.